1. Field of the Invention
This invention concerns quill drives for machine tools, and more specifically, mechanical quill drives.
2. Description of the Prior Art
Feed mechanisms for quill shafts of a type used in machine tools have been in use for many many years. In these mechanisms a quill shaft adapted to rotate a tool spindle is advanced linearly in order to feed the tool into the work.
While quite satisfactory hydraulic design approaches have existed in the past, current trends are for various reasons dictating a "mechanical" approach to the quill unit drive. Such mechanical drives developed to date have various drawbacks.
One such approach involves a screw actuator disposed parallel to the quill shaft which is rotated at two speeds to provide the two-speed advance of the quill shaft. An example of this design is found in U.S. Pat. 3,561,544. Another somewhat similar approach common also in single speed drill press type applications has involved a rack or cam drive of the quill shaft; such as shown in U.S. Pat. No. 2,545,008 and 2,902,891, as well as in German Patent No. 846,807.
These approaches have the common difficulty of excessive runout of the spindle shaft since the feed forces are not aligned with the axis of motion of the quill shaft.
While some attempts to overcome this problem have been made by a power screw arrangement concentric with the quill shaft as exemplified by U.S. Pat. Nos. 2,796,767; 2,975,440; 3,430,524; and 3,139,637, these designs require bearings on a spindle shaft forward of the screw shaft and hence are quite cumbersome, and runout problems may also exist due to the long unsupported screw portion overhung at one end of the quill shaft of the mechanism, and also frictional loads may be quite high.
A concentric ball screw advancing mechanism has been described in U.S. Pat. No. 3,640,147 which would substantially alleviate the friction loads, but this design provides only a single speed feed motion and the screw portion is, similar to the above examples, overhung at one end of the quill shaft necessitating a critical bearing adjustment arrangement to control runout.
The particular means for providing a two-speed feed drive previously proposed have also not been without significant drawbacks.
A common approach seen in some of the aforementioned patents as well as U.S. Pat. No. 3,283,664 has been to provide a pair of motors driving through either a planetary or screw differential gearing. Such gearing is expensive and the controls tend to be quite complex.
Another approach is to provide a variable speed commutated motor driven at variable speed, but such motors as compared to constant speed induction motors require relatively more maintenance (due to the need for brushes), must be relatively larger to provide adequate low speed torque, and the controls tend to be complex and costly.
Other approaches such as shown in U.S. Pat. No. 2,984,159 have used slow, cumbersome, and costly change speed countershaft gearing which tends to necessitate relatively shorter rapid traverse cycles since the change-over cannot be carried out as accurately and as rapidly, with the ultimate result of increased cycle times.
Accordingly, it is an object of the present invention to provide a quill mechanical unit drive which minimizes friction and runout and is relatively compact.